Batch Distillation Unit Operation - Technical Information
The batch distillation unit simulates a batch rectifier comprised of a stillpot, column, condenser and accumulator. Only vapor/liquid separation is modeled. The stillpot, the trays in the column and the condenser are all theoretical stages. Simple distillation (Rayleigh) may be simulated by setting the number of stages to 2 (condenser and stillpot with no column trays) and setting the reflux ratio to zero.
There is no limit to the number of stages that may be considered in a batch distillation model. The stages are considered linked, with the equilibrium vapor from each stage entering the next higher stage and the equilibrium liquid from each stage feeding the next lower stage. The Condenser (or reflux splitter, reflux drum) is designated as stage number one and the stillpot is designated as the highest numbered stage in the model. The pressure for every stage in the model must be defined in the Batch Distillation - Initial Pressure Profile window and the liquid holdup must be defined in the Batch Distillation - Liquid Holdup window.
The basic mode of operation for the distillation, the basic specification for column loading, is constant top vapor rate. The top vapor is the vapor entering the condenser. The specified rate may be changed in a series of steps.
Operating Policy
The overall operating policy of the unit is defined in one or more policy segments. Each segment is made up of one or more actions such as Charge, Draw or Distill and is defined in the Batch Distillation - Policy Segment window. All segments are defined as either sequential or non-sequential in the Overall Operating Policy table on the Batch Distillation Main Window.
The main distillation calculation is comprised of sequential segments which are executed in the order in which they appear in the table unless defined as starting after another specified sequential policy segment. The sequence may also be modified by defining a jump to another sequential segment after Run Until Conditions or with a Go To. There will be one, and only one, sequential segment executing at any time throughout the simulation.
Non-sequential segments run in parallel to the sequential policy executions. They have defined Start conditions and the segment will start when any of the conditions is satisfied. Non-sequential segments are used to define Charges, Draws and dynamic controllers which operate over different time periods from the main sequential segments. There may be more than one non-sequential segment active at the same time.
You must define the maximum time allowed for the simulation in the PRO/II - Batch Distillation main window. You may also define other conditions (temperatures, amounts and purities) which will terminate policy segment executions. Global Stop Criteria apply throughout the simulation and will stop whatever segments are currently executing when any one of the specified conditions is met. Conditions for stopping individual policy segments may be defined in Run Until Conditions within each policy segment.
Feeds and Products
Batch column feed and product streams are added as part of the flowsheet construction in the process flow diagram (PFD). Feed streams are used as sources for the batch column charges. There is no limit to the number of feed streams or charges, but an initialcharge must be made to the stillpot before distillation may be performed. Further charges may be supplied during the operation of the unit. Charges to the stillpot may be instantaneous or continuous, while charges must be continuous to a column tray. A charge to the first stage, the condenser or reflux splitter, is not allowed. .
Charges are taken from the feed streams. Each feed stream may be used for any number of charges. On the Process Flow Diagram, feed streams may be attached to the stillpot port or to the left side of the column.
Liquid products may be drawn from the accumulator, stillpot, or from any stage in the column at any time. Draws from the accumulator or stillpot may be instantaneous. Continuous draws may be taken from any location except the accumulator. An End Cut is allowed only in the accumulator. It instantaneously dumps the total accumulator contents into a product.
In addition, there are three special products:
- the accumulated overhead vapor from a partial or mixed condenser;
- the heel in the stillpot and on the trays at the end of the distillation;
- the contents of the accumulator and reflux splitter at the end of the distillation.
A stream corresponding to each product must be attached to the batch distillation unit on the Process Flow Diagram. Streams for the heel and accumulator residue are required, attached to the red product ports under the stillpot and accumulator. If a partial or mixed condenser is used, attach a stream for the overhead product to the green port on top of the reflux drum.
Separate streams must be attached to the unit for each draw or cut product. Any of these streams may be attached either to the right side of the column or the right side of the reflux drum. Draws from the same stage at different times (different policy segments) require separate flowsheet streams.
Heat Sources and Sinks
Heat sources and sinks apply only to the stillpot and condenser. Heat is supplied to the stillpot and cooling to the condenser to meet the specifications for vapor loading and reflux.
Thermodynamic Systems
Batch distillation only simulates VLE systems throughout the column. In the condenser, the user may choose either VLE or VLLE thermodynamics. The system for the unit is selected in the Thermodynamic Systems Window. It is also possible to define different thermodynamic systems for individual sections of the unit. The reboiler may be modeled as a simple heat exchanger and a separate thermodynamic system may be selected for the hot (utility) side.
Additionally, in the condenser only, the user may choose either VLLE or VLE thermodynamics. The condenser thermodynamics may be different from the rest of the column. Care should be exercised to ensure no discontinuities exist when different thermodynamics methods are used in various parts of the column.
Initialization of Calculations
The batch distillation unit does not simulate heat-up of the equipment or its contents, or the initial loading of the upper stages. To initialize the calculations, a vapor-liquid equilibrium calculation is made for the bubble point of the (combined) initial charge. The vapor composition from this calculation is used as the initial liquid composition in the reflux splitter, and the initial charge composition is used for the liquid in the stillpot. Compositions for the intermediate stages are interpolated.
It is highly recommended, but not required, that the simulation begin with operation at total reflux until steady state. This should simulate conditions at the end of the usual total-reflux startup of a real column.
Related Topics